Furnace



W. K. RANKIN Nov. 13, 1934.

FURNACE Filed Sept. 30. 1933' Patented Nov. 13,1934

UNITED STATES PATENT OFFICE FURNACE William K. Rankin, Lansdowne, Pa., assignor to General Electric Company, a corporation of New York Application september 3o, 1933, serial No. 691,684

My invention relates to furnaces, more particularly to furnaces for heating metals, and it has for its object the provision of an improved device of this character.

l Although not limited thereto, my invention has particular application to furnaces for carrying out certain heating operations for metals, such as soldering, brazing, and annealing in a selected medium, such as an inert gas or a vacuum. My

invention is also particularly applicable to nitriding and like metal heating operations.

In carrying my invention into effect in one form thereof, I provide a heating chamber dened by a wall formed of electrical resistance material 1I which will generate heat by the passage of electric current through the wall'. Suitable electrodes are connected with the end portions of this resistance wall arranged to connect the wall with a suitable source of electrical supply.

I0 It is contemplated thaty the heating operations will be effected in a selected medium, such as a vacuum. A vacuum is created in the chamber in any suitable manner, as by means of a vacuum pump connected with the heating chamber at any suitable point. I

In order to relieve the walls of the heating chamber of atmospheric pressure when operating with a vacuum medium, I surround the major portion of this wall with a vacuum envelope,iwhich preferably will be closed to the heating chamber. The vacuum envelope may also be created by means of a vacuum pump.

Preferably the heating chamber will be surrounded by a layer of suitable heat-insulating material.

For a more complete understanding of my invention, reference should be had to the accompanying drawing, in whichFig. 1 is a vertical sectional view of an electric furnace embodying my invention; and Fig. 2 is a sectional view taken through a cover provided for the furnace.

Referring to the drawing, I have shown my invention in one form as applied to an electric furnace comprising a wall 10 defining an upright cylindrical elongated heating chamber 11. It is contemplated that the Wall 10 will be used not only to receive and support the furnace charge, but also toigenerate the heat necessary to treat the charge. For this purpose, the wall 10 is formed of a suitable electrically conducting resistance material, such as a non-magnetic steel of high electrical resistance and having great mechanical strength at high temperatures. For example, an austenitic steel having a yield point of approximately 49000 pounds per square inch at 68 F., and 4500 pounds per square inch at 1832 F., maybe used. One suitable steel for this purpose has carbon and manganese contents under .16% and .50% respectively, a silicon content over .50%, a chromium content of 6 17.0%-20.0%, a nickel content of 7.00 to 10.0%, and maximum phosphorus and sulphur contents One end of the heating chamber, the lower end, as shown in Fig. 1, is closed by a cover 12 35 having a fluid-tight and electrically-conducting joint 12a with the cylindrical wall of the chamber. This cover may be formed integrally with the side wall, but as shown I prefer to form it as a separate member and to join it with the side wall in any suitable manner, as by welding. The cover 12 is formed of an electrically-conducting material which will have a coeillcient of thermal expansion approximately the same as the material of which the wall 10 is made so as to 5 facilitate making a welded joint between these members.

The other end of the chamber 10 constitutes a charging and discharging opening. This opening is closed by means of a cover member 13. This member is formed of a relatively strong material, such as steel, capable of withstanding high external pressure.

The cover 13 is bolted or otherwise secured to a flange J14 welded on the upper end of the heating chamber 10; as shown, screw-fastening means 15 are provided for this purpose. The flange 14 preferably will be formed as a separate member and will be secured to the heating chamber in any suitable manner, as by welding. Suitable means are provided for effecting a fluid-tight sealed connection between the cover member 13 and the flange 14. For this purpose, the flange 14 is provided with an annular groove 16 which receives an annular depending flange 17 provided on the cover member when the cover member is in place, as shown in Fig. 1. Within the channel 16 is placed a gasket 18 formed of a relatively soft metal, such as aluminum. When the cover mernber 13 is applied and fastened by means of the screw-fastening means 15, the flange 17 will be pressed into the soft aluminum gasket 18 so as to eifect a fluid-impervious seal between the cover and the heating chamber.

The cover member 13 is provided with a passageway 19, as shown in Fig. 2, for conducting a cooling fluid through the cover member, as will be pointed out in greater detail hereinafter; like- Wist?, the flange 14 is provided with a circular 110 nected with the lower inner end wall 12.

The flange 20 is square in form and is arranged to surround the cylinder 10 and to contact the outer surface of its wall. -The flange is secured to this wall in any suitable manner, as by welding. This is preferably accomplished by welding the ange to a reinforcing ring 20a which in turn is welded at its upper and lower ends to the wall 10. It is contemplated that the lange 20 will be cooled by means of a suitable circulating fluid, such as water; to provide for this circulation, the flange is provided with internal passage- Ways. In order to provide for these passageways, the flange preferably will be formed of a pair of square plates 22 and 23, spaced apart somewhat and joined together by means of a pair of concentrically arranged ring-shaped partitions 24 and 25. The inner ring 24 is provided with a ,number of spaced apertures 26. It will be observed by reason of the foregoing construction that a pair of concentrically arranged interconnected channels 2'7 and 28 are provided between the plates 22 and 23. It is contemplated that the cooling fluid will be supplied to the outer channel 28 at one side thereof from Where it will divide and flow in streams in opposite directions around the channel to the other side of this channel. Fluid can ow into the inner channel 27 through the openings 26.

The flange 20 is connected to an external supply lead 29 by means of a suitable copper electrode 30. This electrode, as shown, comprises a pair of plates 31 and 32 spaced apart by means of suitable spacers 33 arrangedat their ends and secured together by means of suitable screw-fastening means 34. The inner screw-fastening means 34 functions to secure the connector member 30 to the ilange plates 22 and 23, as clearly shown in the figure. The ends of the electrode are closed by means of suitable plugs (not shown). The electrode therefore has the form of a closed box. It is contemplated that a cooling fluid will be passed through the electrode while the furnace is operating.

If it is desired, a similar connector member 30a may be provided on the other side of the flange 20 connected to the flange by means of screwfastening means 34a. 'Ihese two electrodes are preferable because by supplying the current to the flange 20 on opposite sides, the ow of current through the wall 10 is equalized.

The lower electrode 21, as shown, is of tubular form and is arranged substantially in the central longitudinal axis of the heating chamber. The upper end of the tubular electrode 21 is electrically connected with the cover member 12 by means of a screw-threaded connection 36, which will be brazed so as to provide a fluid-tight joint, irrespective of the difference in thermal expansion of the members 12 and 21. The outer end of the electrode, as shown, is closed by means of a cover 37 which will be welded or otherwise connected to the tubular member soV as to provide a uidtight joint between these members.

The electrode is connected with electrical supply source by means of a connector member 38 rigidly secured to the electrode by suitable clamping bolts (not shown).

It is contemplated that this electrode also will be supplied with a circulating cooling fluid. For this purpose, fluid is supplied to the electrode by means of a suitable supply conduit 39 which conducts the fluid into the upper end portion of the cylinder, and with an exhaust pipe 40 for conducting the fluid from the electrode.

A false bottom 42 is provided in the lower portion of the heating chamber spaced a relatively short distance from the bottom wall'12. This false bottom 42 may be formed of any suitable material, such as that used to form the wall 10,

and preferably will be welded to the wall 10. This bottom is provided with a plurality of apertures 43 connecting the heating chamber proper with the space under the bottom.

As previously pointed out, it is contemplated that the heating operations will be carried out in the preselected medium which preferably will be a vacuum. In order to create a vacuum in the heating chamber, a suitable vacuum pump 44 is provided. This pump 44 is connected with the heating chamber by means of a conduit 45 which is connected with the pump at one-end and has its other end connected with a second conduit 46, as shown in Fig. 1. The conduit 46 communicates directly with the upper end portion of the heating chamber. It will be understood that the conduit 46 will be connected with the heating chamber by means of a suitable Huid-tight joint 46a, such as a welded joint. The conduits 45 and 46 will be connected together by means of a suitable uid-tight joint 47 which may be arranged similar to the joint between the cover 13 and the flange 14.

In the operation of the furnace thus Afar delos rio

scribed, it will be understood that the charge will be inserted into the heating chamber through the charging opening at its upper end, and that after the charge has been inserted the cover will be closed. Electrical current will be supplied to the resistance wall 10 by means of the electrodes 30 and 2l so that the wall will be heated up by resistance to the passage of current through them. The charge placed upon the oor 42 is brought to substantially the same temperature as the wall 10. After the charge has been treated, the cover 13 may be removed and the charge taken from vthe heating chamber. The pump 44 of course will have been operated so as to eiect the vacuum medium within the heating chamber While the charge was being treated.

The thickness of the wall 10 will be such as to give the desired temperature condition with reasonably low starting and operating currents; it is preferable that the walls be made as thin as possible. In the specific application disclosed wherein the furnace has an internal diameter of approximately 10 inches and a length of approximately 36 inches, it is preferable that the walls be made approximately M3" thick. This wall operating on a 10 volt, 60 cycle alternating-current supply source will maintain a temperature of 1000 C.

The wall-10 of the above dimensions operating at this temperature of 1000 C., will not withstand the full atmospheric pressure without eventually collapsing. In order to removethis pressure from the wall, I have enveloped substantially the major portion of this wall in a vacuum envelope closed both to the heating chamber and to the outside atmosphere. This envelope is provided by means of a cylindrical shell 50 having an internal diameter somewhat larger than the external diameter of the heating chamber and arranged substantially concentrically with the heatlin 1,9so,eas

ing chamber so as to provide between these memebrs.

A fluidtight joint is provided between the flange 20 and the upper end of the shell 50 to render the connection between the heating chamber and the shell fluid-tight, and also to electrically insulate the upper end of the shell 50 from the flange 20. This is accomplished by providing a flange 52 on the upper end of the shell. This flange is mechanically secured to the flange 20 bythe screw-fastening means 34, 34a. Interposed between the flange -52 and the lower surface of the electrode is a suitable electrically-insulating gasket 53, formed of any suitable material, such as fiber. The gasket 53 is placed upon a series of annular channels 54 arranged in the upper surface of the flange'52 so that when the flange is drawn tightly to the flange 20 by means of the screw-fastening means 34, the insulating gasket will be pressed down into the grooves so as to effect a fluid-tight connection. between the flange and the shell 50. The bolts 34, 34a are electrically insulated from the flange 52 by means of insulating bushings 55 surrounding the bolts, as shown, and by means of washers 55a between the heads of the bolts and the flange. The bushings and washers may be formed of flber. The flange 52 is provided with a channel 52a fora cooling fluid.

The shell 50 projects somewhat beyond the lower end of the heating chamber, as shown in Fig. 1, and this lower end of the shell is closed by means of a suitable cover 56, which is provided with a centrally arranged aperture for receiving the electrode. The cover 56 is connected to the shell 50 by means of a fluid-tight joint, which comprises a flange 57 welded to the lower end of the shell 50. The cover is sandwiched between the flange and a ring 58 placed below the cover, as shown. Preferably suitable gaskets 60 formed of any suitable material, such as cork and rubber, will be interposed between the cover 56 and the adjacent surfaces of the flange 57 and of the ring 58. The ring 58 is rigidly secured to the flange by means of suitable screw-fastening means 61. A suitable cooling fluid channel 57a is provided in the flange 57.

A fluid-tight seal is provided between the electrode and the cover. This seal comprises a flange 62 brazed to the outer surface of the electrode and with which a ring 63 arranged on the exterior of the \cover cooperates to sandwich the cover between them. The ring 63 is secured to the flange 62 by means of suitable screw-fastening means 64. As shown, the flange 62 is provided on its lower surface with a pair of flanges 65, while the ring 63 is provided with a circular flange 66 arranged between the flanges 65 so that when the ring 63 is screwed tightly against the cover, the flange 66 will press tightly against the cover on the outside between the flanges which press tightly against the cover on the inside. This arrangement effects a very good fluid-tight joint between the cover and the electrode.

It has been found that the heating chamber when operating will thermally expand somewhat more than will the shell 50 and consequently the lower ends of the -chamber 10 and the shell 50 will move somewhat relatively to each other. To provide for this relative movement,'the cover 56 is arranged to provide an expansion joint between these members. The cover 56 is formed of a flexible material and moreover, as shown, is provided with a bulging or curved portion 67. This bulging portion absorbs any movement that an annular space 51 may take place between the inner and outer walls 10 and 50.

A vacuum is created in the annular space 51 by means of a suitable vacuum pump 68 connected with the annular space by means of conduits 70 and 71.

It has been found that when the furnace 10 is operating at a high temperature there is a considerable heat loss through the vacuu'm in the annular space 51. To overcome this, a relatively heavy layer of thermal insulating material 72 is applied to the external surfaces of the heating chamber 10. This layer 72 substantially fllls the annular space 5l, but preferably a relatively small space 73 will be left between the outer surface a silica asbestos composition, such as used for pipe covering. The flange 74 also will be formed of asbestos.`

The cooling fluid for the flange 20, the electrode 21, the connector members 30, 30a, the cover 13, the flange 52, and the flange 57 may be connectedV into a closed fluid-circulating system. Thus the pipe 39 for the electrode 21 may be connected to a suitable source of cooling fluid supply, such as water. The pipe 40 discharging from the electrode may be connected with one' sideof the channel 57a in the flange 57 by means of a conduit 75. The opposite side of this channel is connected with the right-hand side of the channel 52a in the flange 52 by means of a conduit 76. The opposite side of this channel is connected to the cooling chamber of the electrode 30a by means of a conduit 77, the electrode in turn being connected with one side of the chamber 28 in the flange 20 by means of a conduit 78. The opposite side of this chamber 28 is connected by means of a conduit 79 with the electrode 30. This electrode is connected by means of a conduit 80 with one side of the channel 19 in the cover 13, vas shown, the opposite side of the channel being connected with the channel 19a in the flange 14 by means of a conduit 8l. The system is completed by a discharge conduit 82 which is connected to the other side of the channel 19a.

It will be observed that the cool water which enters the electrode 21 will flow from there to the channel 57a and from this channel will flow 'to the channel 52a, then through the channel 52a to the left-hand connector 30a and thence to the chamber 28 of the flange 20. From this chamber the fluid will flow to the right-hand connector 30, from 4whence it will flow to the channel 19 in the cover. Fromthis channel the fluid will flow to and through the channel 19a to the drain 82. Preferably, the cooling fluid will be forced through the system by means of a suitable pump, or other pressure device (not shown).

An electrically insulating hose 83 is inserted in the conduit 77 so as to insulate the electrode 30a from the furnace shell 5D. Likewise, an insulating hose 84 is inserted in the conduit 80 between the electrode 30 and the cover 13. A flexible hose 85 is inserted in the conduit 81 between the cover 13 and the flange 14.V The flexible hoses 84 and 85 provided for removal of the cover 13.

In the operation of the furnace, the charge to be treated will be placed within the heating chamber on the iioor 42, and the connectors 30 and 38 will be connected to a suitable sourcel of electrical supply, as has been pointed out above. The vacuum maintained in the heating chamber 11 by means of the pump 44 preferably will be as 10W as '.15 to 3 microns of mercury. It is not necessary that the vacuum maintained in the annular space 51 by the pump 68 be so low. This vacuum may be held between 1 and 3 millimeters of mercury. After the charge has been treated it can be removed by taking off the cover 13.

The provision of the vacuum envelope about tle heating chamber, closed both to the heating chamber and to the atmosphere, is an-important feature of my invention. This arrangement prevents a considerable loss of energy from the heating unit that would occur if the heating chamber and envelope were in communication. If these spaces were in communication there would be a considerable loss of pressure in both chambers due to the gas emitted from the usual furnace charge, and becauseof this there would be a considerable loss of heating energy due to the rapid radiation and convection of heat to the outer Walls of the shell 50. This is avoided in my invention by separating the vacuum envelope completely from the heating chamber. Also it is to be noted that air is let into the heating chamber when the cover 13 is removed for the purposeof charging or discharging the furnace. If the shell 50 were in communication with the heating chamber, this air would necessarily ll the annular'space 51, and the insulating material 72 within this space would absorb a great quantity of the air. As a result, it would take considerable time to remove this air from the insulation for a subsequent operation of the furnace; it may take as long aTs a day or two to accomplish this.

A furnace arranged in accordance with my invention can be used conveniently to silver solder or braze together rather complicated parts. Thus flexible leads of copper laminations have been successfully welded so as to form solid ends for connections. Very complicated valves for vacuum work have been made in this way. Moreover, the silver soldering of copper and steel parts can be accomplished without the use of borax or boric acid as a ux.

. While this furnace is quite applicable to brazng, welding, and like processes, it is not limited thereto. Thus it is contemplated that this furnace can be used very eiiiciently for nitriding processes. The nitriding gas used in the process may be led into the bottom of the heating chamber by means of a suitable conduit (not shown) or an ammonia gas may be introduced by the conduit and caused to pass up through suitable copper clips (not shown) to disassociate the ammonia into nitrogen and hydrogen, the nitrogen being used for the nitriding process. This gas then passes around and over the charge and nally passes out of the heating chamber at the top through the conduit 46.

It has been found that the provision of the resistance walls 10 which function both to receive the furnace charge and to heat i-t contributes greatly to lowering the cost of nitriding. The charge is heated uniformly by the walls, and there is very little loss of heat through the insulating layer '72. Moreover, by using the vacuum principle, the residual gases from the metal may be removed before the nitriding gas is generated to nitride the metal.

Moreover, the use of a low voltage, such as 10 volts, is important. If a high voltage, such as 110 volts were used, great care must be used when turning on the current to make sure that suflicient vacuum has been obtained so as to prevent a gaseous electrical discharge across the terminals. Should such a discharge start, it is quite likely that it would break down' into a power arc and destroy the heating element. It is impossible for such an arc to form under any condition of vacuum when operating on a 10-volt supply source.

While I have shown a particular` embodiment of my invention, it will be understood of course that I do not wish to be limited thereto since many modifications may be made, and I therefore contemplate by the appended claims to cover any vsuch modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States, is:

1. A furnace comprising a metal heating chamber, means for heating said chamber, means for maintaining a vacuum in said chamber, a vacuum envelope closed to said heating chamber surrounding the major portion of said chamber and a heat insulating material substantially lling said vacuum envelope.

2. A furnace comprising walls formed of an electrical resistance material defining a heating chamber, electrodes for connecting portions of said walls to an electrical supply source, means for maintaining a vacuum in said chamber, a second chamber closed to the outside atmosphere and to said heating chamber substantially enveloping said heating chamber, means for maintaining a vacuum in said enveloping chamber and a heat insulating material substantially filling the entire second chamber.

3. A furnace comprising wallsformed of an electrical resistance material defining an elongated heating chamber, an electrode connected to the end wall portions of said chamber arranged to connect said walls to a source of electrical supply, means for maintaining a vacuum in said 'heating chamber, walls defining an elongated 120 chamber substantially enveloping said heating chamber closed both to said heating chamber and to the outside atmosphere, thermal insulating material substantially lling the space between said heating chamber and said enveloping chamber, and means for maintaining a vacuum in said enveloping chamber.

4. A furnace comprising walls dening a heating chamber having a charging and discharging opening in one end thereof, means closing the other end of said chamber, means for heating said chamber, walls spaced from said rst walls dening a second chamber enclosing substantially all of said heating chamber with the exception of said charging and discharging end, means securing the ends of the walls of said chambers together at the charging and discharging end of said heating chamber with a huid-tight joint, an expansion member at the other ends of said chambers, means connecting said expansion member with a fluid-tight joint to said latter end of the walls of said outer chamber and means connecting said expansion member with a fluid-tight joint to the latter end of the walls of said heating chamber whereby a fluid-tight expansion joint i is effected between said walls, and means for maintaining substantially vacuums in said chambers.

5. A furnace comprising walls made of an electrical resistance material dening an elongated heating chamber having a charging and discharging opening in one end thereof, a conducting ilange encircling and contacting the end of said chamber adjacent said charging and discharging opening, an electrode connected to said flange, a second electrode arranged substantially in the longitudinal axis of said chamber connected to the other end of said chamber, walls spaced from the walls of said heating chamber defining a second elongated chamber, a fluidtight joint between one end of said second chamber and said ilange, a metallic expansion joint between the other end of said second chamber and said second electrode, and means for maintaining substantially vacuums in said heating chamber and in the space between said heating chamber and said second chamber.

6. A furnace comprising Walls dening a pair of substantially concentrically arranged cylinders, means for closing the inner end of the inner cylinder, the Walls of the said inner cylinder being made of an electrical resistance material, a conducting flange encircling and contacting the inner cylinder adjacent its outer end, a uidtight joint between said flange and the corresponding end of said outer cylinder, an electrode connected to said flange, a second electrode arranged longitudinally of said cylinders connected to the inner closed end of said inner cylinder, a metallic expansion joint connecting the inner end of said outer cylinder and said last-named electrode, thermal insulating material substantially filling the space between said cyinders, means for maintaining a. vacuum in said inner cylinder, and separate means for maintaining a Vacuum in the space between said cylinders.

7. A furnace comprising walls formed of an electrical resistance material deiining a heating lchamber, electrical supply electrodes connected to said walls, means for maintaining a vacuum in said chamber, a second chamber closed to the outside atmosphere and to said heating chamber substantially enveloping said heating chamber, means for maintaining a vacuum in said e n veloping chamber, a relatively low voltage supply source connected to said electrodes, and heat insulating material substantially filling said enveloping chamber.

WILLIAM K. RANKIN. 

